Cleaning processes in a semiconductor manufacture field or the like use ultrapure water as cleaning water. The ultrapure water should have a remarkably low content of particles, organic substances or inorganic substances which may cause cleaning troubles. The ultrapure water is usually required to have a water quality having electrical resistivity of higher than 18.2 MΩ·cm and having contents of particles of less than 1 particle/mL, bacteria of less than 1 colony/mL, TOC (Total Organic Carbon) of less than 1 μg/L, silica of less than 1 μL, metals of less than 1 ng/L, and ions of less than 10 ng/mL.
Ultrapure water produced in an ultrapure water producing apparatus is sent to a point of use via sending piping, and residual ultrapure water which is not used in the point of use is recycled to the ultrapure water producing apparatus via returning piping.
FIG. 19 shows a conventional ultrapure water producing system wherein primary pure water stored in a subtank 71 is sent via a pump 72 to a heat exchanger 73 to be heated or cooled, and the water is treated by a UV (ultraviolet) oxidation apparatus 74, an ion exchange device (deionizer) 75, and a UF (ultrafiltration) membrane device 76 to produce ultrapure water. The ultrapure water is sent to a point of use 77 via piping (feeding line) 86. A part of the ultrapure water is used in the point of use 77, and the residual ultrapure water which is not used in the point of use 77 is returned to the subtank 71 via piping (return line) 87.
The primary water is produced by treating raw water such as industrial water, well water and municipal water by a pretreating device such as a coagulation device, and then treating the pretreated water by a first RO (reverse osmosis) device, an ion exchange device (deionizer) containing an anion exchange resin and a cation exchange resin, and a second RO devise in this order.
The point of use 77 includes a cleaning device for cleaning an object such as semiconductors or others, and may further have pipes and nozzles.
The ultrapure water is circulated continuously in the system so as to remove impurities dissolved from constituent materials of pumps, heat exchangers, or pipes of the system and to keep high purity of the ultrapure water. The constituent materials of the pumps, the heat exchanges or some portions of the pipes are usually SUS.
The system for producing ultrapure water is sterilized periodically by supplying water containing bactericide (hereinafter referred to as “bactericide water”) to flow through the system in order to reduce the number of bacteria in the ultrapure water fed to the point of use 77 down to less than the number of bacteria required for the level of the above-mentioned water quality requirement. Hydrogen peroxide has been employed as the bactericide, and the system has been sterilized conventionally by the following procedures i) through iii). The ion exchange device (deionizer) 75 is equipped with a bypass line 84A in order not to supply hydrogen peroxide solution therethrough for preventing ion exchange resins from oxidation or deterioration.
i) After the system for producing ultrapure water is stopped running, a concentrated solution of hydrogen peroxide is fed into the subtank 71 via a line (piping) 88 so as to fill the subtank 71 and all the lines with bactericide water containing hydrogen peroxide in a content of about 0.1 to 3%. The bactericide water is circulated through the whole system by the pump 72. Concretely, the bactericide water is circulated through the subtank 71, the line 82, the pump 72, the heat exchanger 73, the line 83, the UV oxidation apparatus 74, the line 84, the bypass line 84A bypassing around the ion exchange device 75, the line 85, the UF membrane device 76, the line 86, the point of use 77, the line 87, and the subtank 71, in this order (hereinafter, this process is referred to as “circulation process”). In this process, the bactericide may be introduced into the subtank 71 through a manhole thereof, or added therein by using an ejector or a pump.
ii) The bactericide water is retained in the system for predetermined period of time after the pump is stopped (herein after, this process is referred to as “steeping process”).
iii) The bactericide water in the system is drained out via a drainage line 89 of the subtank 71, and then the subtank 71 is rinsed with water, after that, the system is further rinsed with ultrapure water by using the pump 72 until the bactericide is not detected in the water (hereinafter, this process is referred to as “rinsing process”).
After the rinsing process iii), the operation of the system for producing ultrapure water is started again.
Since a density of semiconductor integrated circuits has been increased, ultrapure water for cleaning thereof has been required higher purity, and the ultrapure water system is required to produce ultrapure water with high purity even just at restart of operating the system after its sterilizing treatment.
The inventors have found however that the ultrapure water produced by the system contains metals in a high concentration at reoperation of the system after the sterilizing treatment, and the concentration of metals is kept high for a long period thereafter. Concretely, the ultrapure water sometimes contains 0.1 to 1 ng/L of metals such as Fe, Cr, Ni after the sterilizing treatment, and, in particular, Fe is detected even after 1 week has lapsed from reoperation of the system.
The inventors have investigated causes of the high concentration of metals in the ultrapure water at the reoperation of the system, and found the causes as follows: Metals dissolve into ultrapure water from consistuent materials of pumps, heat exchangers, pipes and so on during operation of the system. Metals also remain in a reserve pump which is out of operation. These metals are oxidized by hydrogen peroxide and dissolve into the water. The dissolved metals form particles of metal hydroxides. The particles thus formed are captured by a membrane in a membrane separation device arranged at the last stage of the ultrapure water producing apparatus. The captured particles re-dissolve into the ultrapure water after hydrogen oxide is drained out of the system during a rinsing process.